issue a user warning if random is None

mesa/agent.py

@@ -120,6 +120,11 @@
             random (Random): the random number generator
         """
         if random is None:
+            warnings.warn(
+                "Random number generator not specified, this can make models non-reproducible. Please pass a random number generator explicitly",
+                UserWarning,
+                stacklevel=2,
+            )
             random = (
                 Random()
             )  # FIXME see issue 1981, how to get the central rng from model

mesa/experimental/cell_space/cell_collection.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import itertools
+import warnings
 from collections.abc import Callable, Iterable, Mapping
 from functools import cached_property
 from random import Random
@@ -45,7 +46,12 @@ def __init__(
         self._capacity: int = next(iter(self._cells.keys())).capacity
 
         if random is None:
-            random = Random()  # FIXME
+            warnings.warn(
+                "Random number generator not specified, this can make models non-reproducible. Please pass a random number generator explicitly",
+                UserWarning,
+                stacklevel=2,
+            )
+            random = Random()
         self.random = random
 
     def __iter__(self):  # noqa

mesa/experimental/cell_space/discrete_space.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import warnings
 from collections.abc import Callable
 from functools import cached_property
 from random import Random
@@ -44,6 +45,11 @@ def __init__(
         self.capacity = capacity
         self._cells: dict[tuple[int, ...], T] = {}
         if random is None:
+            warnings.warn(
+                "Random number generator not specified, this can make models non-reproducible. Please pass a random number generator explicitly",
+                UserWarning,
+                stacklevel=2,
+            )
             random = Random()  # FIXME should default to default rng from model
         self.random = random
         self.cell_klass = cell_klass